Mobility, defined as an individual's ability to move about effectively in his or her surroundings, is fundamental to independence and quality of life. Difficulty with mobility (mobility disability) is highly prevalent in older adult populations, with negative consequences for independence and social isolation. Recent models of disability draw attention to the role of environmental factors that can interact with an individual's underlying impairments or capacities to impede or enhance a person's ability to be independent in mobility. For example, living on a street without continuous, barrier-free sidewalks may hinder mobility for a person living with pain or leg weakness due to arthritis. Using secondary analyses with data from the recently launched National Health and Aging Trends Study (NHATS), this proposal aims to further our understanding of the dynamic nature of the disablement process. Funded by the NIA, NHATS is a nationally representative study designed to support the investigation of disability trends and dynamics in later life. NHATS uses innovative measures of disability and function, captured both objectively (through physical performance measures) and subjectively (through survey questions), to tease apart the various elements along the disablement process. In addition, interviewer observations of each respondent's residential location provide objective data on the barriers and facilitators in the surrounding built environment. Drawing on this unique set of measures in a large, nationally representative sample, this proposal seeks to understand how characteristics in the built environment may increase the risk of, or buffer against, mobility disability and participation restriction in older adults. The relationship between the built environment and mobility disability is complex, incorporating interactions between individual and environmental factors. A better understanding of the dynamics in this relationship is critical for the planning and development of 'age-friendly' environments, which allow older adults to 'age in place'.